Ms Gillard, who relied on the union movement to fend off leadership pressure from Kevin Rudd, charged taxpayers more than $15,000 to fly the pair to the G20's related trade union conference, the L20.

The release of Senate documents showing the cost of the trip comes at an inconvenient time for the union movement. The government has ordered a royal commission into union corruption - a move Labor frontbenchers have described as politically motivated.

The Gillard government paid for flights and accommodation for two ACTU officials to attend the proceedings on July 17 and 18 last year. The decision to send them was Ms Gillard's, according to documents released by the Department of the Prime Minister and Cabinet to the Senate.

Under orders from Ms Gillard, the department entered into an agreement with the then Department of Education, Employment and Workplace Relations under which the latter would be given $50,000 ''from which it could draw funding to reimburse the ACTU for the costs of attending meetings covered by the then prime minister's decision''.

Only $15,108.60 was used. The remaining $34,891.40 was returned to the Department of the Prime Minister and Cabinet.

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An ACTU spokeswoman said: ''There is a long-standing precedent for governments of all persuasions to support representatives of government, business and civil society, including trade unions, to attend joint international meetings where there is a formal role for non-government parties.''

While corporate lobby group the Business Council of Australia has a policy of not accepting government money for travel to conferences, it is believed other business and community representatives have done so.

Asked if there was a precedent for the prime minister's department to fund unionists to attend such conferences, the deputy secretary of the G20, Heather Smith, told the Senate: ''I am not aware that we have but I will … have to take that on notice.''

A spokesman for Ms Gillard declined to comment and the Department of the Prime Minister and Cabinet did not answer more questions.